The present invention relates to an adsorption process for recovery of solvents.
In the painting and printing industries the paint or ink is dried by evaporating the solvent with hot air. The solvent laden air contains typically 2-12 grams of solvent in a standard cubic meter (g/scm) and 6-24 g/scm water vapor. This air is passed through activated carbon. Most of the solvent and some of the water is adsorbed on this carbon and the purified air is vented or reused for paint drying. The adsorption can be carried out with a moving bed or fixed beds of activated carbon. Horizontal, vertical or radial fixed beds may be utilized as well. The solvent adsorbed on the activated carbon can be desorbed by heating and stripping either with steam or a hot inert gas with a low oxygen content.
A disadvantage of the desorption with steam is, that the recovered solvents contain large amounts of water which form azeotropic mixtures with many solvents.
When the desorption is carried out with hot inert gas, the solvent is recovered by chilling the circulating inert gas. Water is also being desorbed during the heating and stripping with hot gas and the condensate recovered by chilling contains 4-10 wt % of water. If molecular sieves are used to remove the water from the gas before its chilling, a solvent with less than 0.3 wt % water can be recovered. The regeneration of the activated carbon bed is completed by cooling the hot bed with cold gas, preferably to below 90.degree. C., to exclude the ignition of solvent or carbon when air is again passed to the bed. The regeneration thus includes a heating and a cooling step. In adsorption processes for solvent recovery with inert gas regeneration both the heating and the cooling of the beds is carried out in closed loops: The gas leaving one end of the bed is cooled and/or heated in suitable exchangers to be returned to the other end of the same bed. A blower is provided for the gas circulation.
The disadvantages of this process are the additional cost of the molecular sieves beds, the associated switching valves and other equipment, such as necessary to regenerate the molecular sieves. Another more important disadvantage is that during the cooling of the bed a substantial amount of solvent from the circulating inert gas leaving one end of the bed cannot be removed by chilling. Typically the gas leaving the chiller at around -10.degree. C. still contains 30-100 g/scm solvent. This solvent is readsorbed on the other end of the bed, thereby significantly reducing the available adsorptive capacity of the bed. Because of the lower adsorptive capacity the time of adsorption must be shortened and consequently the time of regeneration as well. To complete the regeneration (heating and cooling) in a shorter time, a larger flowrate of inert gas must be applied, which requires more heating, more refrigeration for chilling the gas and a larger blower: in general more fuel and electric power and more expensive equipment. In the process using fixed beds, several fixed beds are installed and while one or several beds are cyclically in the phase of adsorption one bed is being regenerated. Adsorptive separations using heating for desorbtion are called thermal swing processes.
A way to reduce the amount of required heat is to begin the cooling step before the entire bed reaches the maximum temperature. The cold gas is taking up heat at the inlet end of the bed and depositing it further ahead, pushing the hot front toward the outlet of the bed. This last method called "thermal pulse" is used with preference in the field of solvent recovery. Its advantage is, that at any time, only one bed is in regeneration. However this method does not efficiently solve the problems of the solvent reloading into the bed and the drying of the gas.
Another way to save heat is the following: When a hot adsorbent bed is cooled, the gas exiting from the bed is warm. It can be further heated and used to heat another bed. In this way the heat contained in one bed is partly recovered to heat another bed. This possibility is sometimes used in the field of drying of natural gas or other gases.